Hormones' role in human sexuality

January 19, 2009|By DAVID PERLMAN / San Francisco Chronicle

In spring a young man's fancy lightly turns to thoughts of love ... but whither when the year is young?

It matters not, for as long as the hormones are in tune, love can bloom at any time, say scientists who study the genetics and neurobiology of animals whose family lives shed new light on human sexuality.

Larry J. Young, a Georgia neurobiologist, studies the genes and hormones of the cute but often pestiferous little beasts called prairie voles, which mate and bond for life.

Those genes and hormones exist in humans, too, and in a uniquely literate essay in Thursday's issue of the journal Nature, Young points to the role they play in animals and humans.

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"Poetry it is not. Nor is it particularly romantic. But reducing love to its comprehensive parts helps us understand human sexuality and may lead to drugs that enhance or diminish our love for another," he said.

Young, a professor in the psychiatry department at Emory University in Atlanta and the Yerkes National Primate Research Center there, has discovered that two closely related peptide hormones called oxytocin and vasopressin play powerful roles in both animal and human sexuality.

"I call oxytocin the motherly hormone," Young said in an interview, "because its release in the body of female voles is involved in uterine contractions, in lactation and in the mother's early bonding with vole babies. It's also the hormone responsible for lifelong pair bonding between males and females.

"Vasopressin is closely related to oxytocin, but it's dependent on testosterone -- so it's the macho version of oxytocin."

Scientists say Young's research of togetherness in prairie voles is proving valuable to understanding many disorders in the human condition.

"Oxytocin is the hormone of monogamy," said neuropsychiatrist Louann Brizendine, who directs the Women's Mood and Hormone Clinic at the University of California-San Francisco. "If you give men nasal squirts of oxytocin, it increases their trust in others, their ability to be affectionate -- it brings out their feminine side, is how I'd put it.

"Years ago a man I was seeing said to me, 'Give me some of that oxytocin -- I just want to love someone!' "

Because monogamy and love are examples of powerful human interactions, Brizendine believes the hormones involved could also prove important for people with disorders like autism, Asperger's syndrome and even schizophrenia, because the symptoms mean they can rarely find close relationships.

Young's primary research shows the effects of the hormones on prairie voles, but he also discovered that oxytocin has similar effects on mountain voles, a different species that are not monogamous and do not pair-bond for life. When mountain voles are exposed to doses of oxytocin in the lab, the males will bond with females in a close and sometimes lifetime relationship like their cousins, Young said.

The varied genes that trigger the release of oxytocin and the testosterone-dependent vasopressin in both voles and humans, Young said, have obviously had a long evolutionary history, as it is also found in other, more primitive pair-bonding animals -- even though monogamy is extremely rare throughout the animal kingdom.

"In all my work," Young said, "I'm trying to understand how we humans interact with each other -- to understand the genetics and biochemistry of the social brain, and how it may go wrong in profound disorders like autism and schizophrenia."

Brizendine called Young's work "deeply important because it clearly correlates the phenomenon of pair-bonding in his voles with monogamous relations in humans."

As for the vasopressin, Brizendine said, one variation in the gene that controls the hormone can make a man "more prone to long-term bonded relationships," while a man with another variation "will be prone to infidelity."

Dacher Keltner, a psychologist and director of the Berkeley Social Interaction Laboratory at UC-Berkeley, calls Young's vole research -- along with work by Sue Carter at the University of Maryland and Thomas R. Insel at the National Institute of Mental Health -- "unbelievably influential in showing us that there's a physical, biological basis to long-term monogamy, love, devotion and feelings of trust with each other."

The new awareness of the role of hormones in human relations is underscored in new books being published this year by both Keltner and Brizendine. "Born to Be Good: The Science of a Meaningful Life" by Keltner is due out next week, and "The Male Brain" by Brizendine is scheduled to be published in September.

Oxytocin, meanwhile, is sold commercially as a "trust hormone" to be used in nasal sprays. One ad claims it can "produce significant increase in charisma, which creates a highly trustworthy appearance," while another calls its brand "a great female seducer."

But Young, Brizendine and Keltner all agree that it's much too early to be peddling the hormones that way.

"We're only at the beginning of our laboratory studies, and people should be extremely careful," Young said.